Miniature microphones, such as those used in hearing aids, convert acoustical sound waves into an electrical signal which is processed (e.g., amplified) and sent to a receiver of the hearing aid. The receiver then converts the processed signal to acoustical sound waves that are broadcast towards the eardrum.
In one typical microphone, a moveable diaphragm and a rigid backplate, often collectively referred to as an electret assembly, convert the sound waves into the audio signal. The diaphragm is usually a polymer, such as mylar, with a metallic coating. The backplate is usually a charged dielectric material, such as Teflon, laminated on a metallic carrier which is used for conducting the signal from the electret assembly to other circuitry that processes the signal.
The backplate and diaphragm are separated by a spacer that contacts these two structures at their peripheries. Because the dimensions of the spacer are known, the distance between the diaphragm and the backplate at their peripheries is known. While the centers of the diaphragm and backplate are separated by a distance that is determined by the distance of separation at their peripheries, the equilibrium separation distance at their centers is also a function of the tension on the diaphragm and the electrostatic forces acting on the diaphragm due to the charge on the backplate. Because the polymer in the diaphragm expands as a function of relative humidity (i.e., hygroscopic expansion) and, thus, its tension changes, the relative humidity of the ambient air affects the equilibrium separation distance. Further, the acoustical compliance of the diaphragm increases with an increase in humidity.
Thus, prior art microphones have a humidity coefficient that affects the sensitivity of the microphone. The sensitivity of the microphone is defined as the output voltage amplitude as a function of the input sound pressure amplitude, and is generally expressed in dB (decibels) relative to 1 V/Pa. The humidity coefficient of the sensitivity is defined as the sensitivity change due to a humidity change, and is expressed in dB per % relative humidity. The humidity coefficient of the sensitivity is a function of both the change in the distance between the diaphragm center and the backplate due to hygroscopic expansion and the change in the diaphragm's acoustical compliance.
A need exists for a microphone that has a reduced humidity coefficient so as to have enhanced performance over a wide range of ambient relative humidity conditions.